Design of dynamically tunable photonic crystal mirrors in THz free electron lasers

A 1-D photonic crystal (PC) mirror of the optical resonator for THz free electron lasers (FELs) is designed theoretically with central frequencies from 1 to 3 THz. The relationship between the reflection spectrum and structural parameters of 1-D PC, such as dielectric thickness, number of layers, and refraction index, is studied. A systematical method to design PC mirrors with high reflectivity is proposed. The out-couplers in the presence of defect layers in PC structure is simulated. It is shown that the out-coupling ratio of the resonator is dynamically tunable by careful adjustment and design of the defect layers. A practical tuning method for THz FELs using PC mirrors is proposed. Moreover, the shift of the whole reflection spectrum is also viable by adjusting the dielectric layer thickness, which extends the tuning range of high reflection PC mirrors and out-couplers.